FIG. 1 shows a conventional variable displacement turbocharger to which the invention is applied. In the turbocharger, turbine and compressor housings 1 and 2 are integrally assembled through a bearing housing 3 by fastening bolts 3a and 3b, a turbine impeller 4 in the turbine housing 1 being connected to a compressor impeller 5 in the compressor housing 2 by a turbine shaft 7 rotatably supported via a bearing 6 in the bearing housing 3.
As shown in FIG. 2 in enlarged scale, the bearing housing 3 is provided, on its turbine housing side, with an exhaust nozzle 9 by which the exhaust gas introduced into a scroll passage 8 in the turbine housing 1 is guided to the turbine impeller 4.
The exhaust nozzle 9 comprises front and rear exhaust introduction walls 10 and 11 on sides of the bearing and turbine housings 3 and 1, respectively, integrally assembled together with a required distance between them by, for example, three fixing members 12 arranged circumferentially. Upon assembling of the turbine and bearing housings 1 and 3, an attachment member 13 fixed on a front surface of the front wall 10 (a side surface of the bearing housing 3) is clamped by the housings 1 and 3 to fix the exhaust nozzle 9. Upon the assembling, the exhaust nozzle 9 is positioned with respect to the bearing housing 3 by a positioning pin 14.
Annularly arranged between the front and rear walls 10 and 11 are a plurality of nozzle vanes 15. In FIGS. 1 and 2, each of the nozzle vanes 15 is dually supported such that the nozzle vane 15 has vane shafts 16a and 16b fixed to opposite sides of the vane 15 and extending through the front and rear walls 10 and 11, respectively.
In FIG. 1, reference numerals 17a, 17b, 17c and 17d designate a linked transmission mechanism for control of opening angle of the vanes 15 through the vane shafts 16a; and 18, a scroll passage formed in the compressor housing 2.
Provided between the turbine housing 1 and the rear wall 11 of the exhaust nozzle 9 is a gap 19 which is unwanted by nature and which is however provided for countermeasure to, for example, possible thermal deformation of the turbine housing 1 between during being hot and during being cold and possible variations in accuracy of parts to be assembled.
The gap 19 may disadvantageously cause the exhaust gas in the scroll passage 8 to vainly leak to a turbine impeller outlet 20. Thus, in order to block the gap 19, it has been proposed to arrange sealing piston rings 21 between an outer periphery on a downstream extension 11′ of the rear wall 11 and an inner surface 1′ of the turbine housing 1 confronting the extension 11′ so as to prevent the gas leakage and absorb thermal deformation (see Patent Literature 1).
In Patent Literature 1, as shown in FIG. 2, formed on the outer periphery of the extension 11′ of the rear wall 11 is an annular groove 22 into which generally two sealing piston rings 21 are inserted with their closed gaps or cutouts being not aligned or overlapped with each other, thereby providing a sealing device 23. The piston rings 21 are pressed at their outer peripheries against the inner surface 1′ of the turbine housing 1 by spring force of the piston rings themselves to prevent the gas leakage.
[Patent Literature 1] JP 2006-125588A